This invention relates generally to optical disc manufacturing and more particularly, to the manufacture of multilayer optical discs.
Multilayer discs are now being produced that are an assembly of two discs, each with its own data layer of grooves or pits representing recorded and stored data or information. Referring to FIG. 5, a composite disc 18 is formed of a first, bottom disc 20 normally having a first, lower, sputtered semi-reflective data layer or surface 22. A read laser either focuses on the first surface 22 or, alternatively, penetrates through the first surface 22 and focuses on a second, data layer or surface 24 on a second, top disc 26. The distance between the two surfaces or focal planes 22, 24 is made up of a transparent bonding layer 28 joining the two discs 20, 26 together. Industry DVD specifications require that the transparent bonding layer be in the range of from 40 microns to 70 microns. Other specifications may be required to comply with other specific formats.
The composite disc 18 is formed from two discs that are normally manufactured using an injection molding process. An ultraviolet, curable, liquid resin, for example, lacquer, that functions as an adhesive is first deposited on a lower rotating disc 20. Upon striking the rotating disc surface 22, the resin spreads across the disc surface 22 toward the inner centerhole 30 and the outer disc circumference 32. However, the resin that first contacts the disc surface 22 will have experienced substantial spreading as the last of the resin is deposited on the surface 22 of the disc 20. Thus, before the final resin deposited spreads, the first resin deposited may reach and spill over the inner hole 30 and/or the outer circumference 32 of the disc 20. Any resin that does spill off of the disc 20 must somehow be handled and contained which adds cost to the process in terms of machine design, maintenance and the cost of the resin.
The construction of the composite disc 18 continues by placing the other disc 26 on top of the resin coating on the upper surface 22 of the first disc 20, thereby forming the multilayer disc 18. The multilayer disc assembly 18 with the uncured resin layer therebetween is subjected to a high speed spinning process to achieve a desired thickness of the intermediate laquer layer. After the spinning process, the resin is cured; and the multilayer disc construction process is complete.
The above spinning process presents several challenges with respect to consistently manufacturing a high quality multilayer disc. First, the spinning process has no process feedback and thus, is not subject to precise control from one disc to another. Further, the forces created by the spinning create a nonuniform resin thickness profile across the resin layer. The resin thickness profile changes from a thinner layer close to the centerhole 30 of the multilayer disc 18 to a thicker layer near its outer circumference 32. The nonuniform resin layer between the discs 20, 26 does not permit a true noise-free replay of the disc.
To hold the thickness of the resin layer within specifications, the resin layer of a finished multilayer disc is measured at many different locations. Those measurements are statistically processed, and the spinning process is adjusted over a period of time, as required, to maintain the resin layer within desired thickness specifications. Many manufacturing environments strive for a thickness specification that is more strict than the about 40-70 microns thickness required by the industry specification. And, the known manufacturing processes can lead to the production of out-of-specification discs while the spinning process is being tuned to hold the resin layer thickness within specification.
Thus, the current multilayer disc bonding process presents several opportunities for improvement. First, there is no real-time control of the thickness of the resin bonding layer between the discs. Second, since the resin is continuously deposited over a single rotation, the first resin deposited reaches the inner hole and the outer edge sooner than the later deposited resin. Third, control over the thickness of the resin layer is further complicated because both of the discs are flat; and therefore, the resin flows freely in all directions and may flow into the inner hole and/or over the outer edge of the disc.
Consequently, there is a need for an improved process for joining two optical discs to form a multilayer disc with an intermediate bonding fluid layer that has a more predictable thickness profile.
The present invention provides an improved optical disc for use in a multilayer disc that provides a consistently high quality noise-free playback of data on the disc. Further, in the manufacture of a multilayer optical disc, the improved optical disc of the present invention can be adhered to another optical disc with a bonding layer thickness that is reliably and repeatably maintained within specification. Therefore, multilayer optical discs are manufactured with a minimum of scrap and thus, manufacturing costs are reduced. Further, the repeatably consistent bonding layer thickness provided by the present invention has the advantage of being able to consistently read data through the bonding layer.
According to the principles of the present invention and in accordance with the described embodiment, the invention provides an optical disc for use in the manufacture of a multilayer optical disc. The optical disc has a first side with a data layer and at least one projection extending outward from the first side. The projection separates the first side from another optical disc used in the manufacture of the multilayer optical disc. The projection is effective to maintain a known and constant bonding layer separation between this optical disc and another optical disc bonded thereto in the manufacture of the multilayer optical disc. Maintaining a repeatably consistent bonding layer improves the consistency of the reading of data from the disc.
In one aspect of the invention, the projection is comprised of a first projection located between a center hole and the data layer and a second projection located between the data layer and an outer circumferential edge of the optical disc. In another aspect of the invention, the first and second projections extend outward from the first side a distance of about 40-70 microns. The use of two projections radially separated on the first side of the optical disc provides a better control over the consistency of the thickness of the bonding layer. In addition, the inner projection substantially stops the migration of air from the centerhole into the bonding layer during a spinning process. Thus, the present invention provides a more consistent and pure bonding layer thickness across the whole diameter of the resulting multilayer optical disc. Such consistency further improves the noise-free readability of data from the optical disc.
In another embodiment, the present invention provides a multilayer optical disc having a first optical disc with a first side and a second optical disc with a first side facing the first side of the first optical disc. An adhesive extends between the first sides of the first and second optical discs, thereby bonding the first and second optical discs together to form the multilayer optical disc. A spacing bridge is located between, and contacts, the first sides of the first and second optical discs. The spacing apparatus separates the first and second optical discs by a distance that is substantially equal to a desired thickness of the adhesive. Again, the multilayer optical disc has a more consistent bonding layer thickness across its whole diameter which further improves the noise-free readability of data from the multilayer optical disc.
In another embodiment, the present invention provides a method of making a multilayer optical disc by first, applying a bonding agent to a first side of a first optical disc during substantially a single rotation of the first optical disc. Next, a first side of a second optical disc is placed on top of the first side of the first optical disc; and the first side of the second optical disc is supported on a spacing bridge extending between the first and second optical discs. The first and second optical discs are spun to spread the adhesive to a uniform, desired thickness therebetween.
These and other objects and advantages of the present invention will become more readily apparent during the following detailed description taken in conjunction with the drawings herein.